Demand for communication traffic has dramatically increased with the progress of multimedia networks. Optical transmission devices, which multi-relay-amplify optical signals using optical amplifiers that use erbium doped fibers (EDFs) as amplification media, play a great role in marketizing such communication traffic.
For example, in an optical communication system, a transmitting end wavelength-multiplexes and outputs optical signals that have a plurality of wavelengths, and the output wavelength-multiplexed optical signal is propagated through a transmission path. For example, in the transmission path, optical amplifiers are inserted and disposed at regular intervals, and hence the loss of the transmission path is compensated. It may be assumed that the number of signal wavelengths of the optical signal propagated through the transmission path and the wavelength allocation of the optical signal arbitrarily vary.
In Japanese Laid-open Patent Publication No. 11-112434 and Japanese Laid-open Patent Publication No. 2001-352119, examples of the configurations of optical amplifiers used in such an optical transmission system as described above are disclosed. When amplifying a signal light, an EDF generates amplified spontaneous emission (ASE). When the number of signal wavelengths is small, e.g., one wave is used, the amount of ASE power included in output power becomes measurable at the time of the control of optical amplification.
As exemplified in Japanese Laid-open Patent Publication No. 11-112434, previously, constant gain control for the EDF has been performed in view of the ASE power generated in the EDF. For example, prepared information of the ASE power generated in an optical amplifier is introduced as a correction term used at the time of gain calculation. In addition, a signal gain is detected on the basis of monitoring values of input power and output power and the above-mentioned correction term, and hence constant gain control is performed.
Incidentally, in recent years, a transmission distance per one span has been increasing, and the output level of an optical amplifier has been rising. When the output level of the optical amplifier rises, a physical phenomenon called spectral hole-burning (SHB) occurs in the EDF. When the SHB occurs, a phenomenon in which only a gain in the vicinity of a signal wavelength is depressed appears. When the number of signal wavelengths for which ASE correction is most important is small, the phenomenon may become pronounced.
In addition, in an optical amplifier under the constant gain control, the phenomenon of gain depression due to the SHB produces a different effect on the optical amplification of an optical signal wavelength when ASE is mixed with an optical signal in input light or when ASE is not mixed.
In the related art, while the amount of ASE power generated in an amplifier itself is subtracted at the time of constant gain control and hence the realization of high accuracy is secured, the constant gain control has not been coupled with the amount of gain depression due to the SHB the effect of which differs in response to ASE included in input light.
In the optical communication system, it may be assumed that, in an optical amplifier disposed on an upstream side near a transmitting end, the amount of ASE mixed in an optical signal is small, and the amount of gain depression is relatively small. However, it may be assumed that, in an optical amplifier disposed on a downstream side, the amount of ASE mixed in an optical signal becomes large, and the amount of gain depression also becomes large. Therefore, it may be assumed that the level of the optical signal does not reach a target level.